.backtop {
  position: fixed;
  left: 50%;
  bottom: 100px;
  margin-left: 650px;
  cursor: pointer;
}
.backtop i{
  display: block;
  width: 28px;
  height: 67px;
  background: url(../images/backtop.png) no-repeat 0 0;
}
.backtop i:hover{
  background-position: -38px 0;
}